1. Field of the Invention
This invention relates generally to a hydraulic system having a hydraulic accumulator for an automatic transmission, and more particularly to an accumulator that maintains hydraulic pressure for immediate engagement of the transmission following an engine restart.
2. Description of the Prior Art
In order to improve the fuel economy of vehicles that use an internal combustion engine for propulsion, some vehicles employ a stop-start strategy wherein, when the vehicle is stopped at a traffic light, for example, the engine is automatically turned off. Then when the driver releases the brake pedal to apply pressure to the accelerator pedal, the engine is automatically started to allow the vehicle to accelerate. For such vehicles that also have an automatic transmission, a need arises to maintain hydraulic pressure in the transmission so that the transmission can engage immediately after the engine is automatically restarted, which will allow the vehicle to accelerate without hesitation.
In order to address this need, some vehicles employ an electric pump for pressurizing the hydraulic fluid rather than a conventional engine-driven pump. While this allows for the maintenance of hydraulic pressure while the engine is shut off, it may be more costly and require more packaging space than is desired. It also may require an additional battery to power the electric motor. A system of this type may also require continuous operation even when a pressure charge is not required due to a delay between a pump-motor startup and hydraulic pump prime time.
Another solution is to use a spring-loaded piston accumulator with a mechanical latch. This approach calls for control system redesign to package it internally. Such a spring-loaded accumulator may require one hundred percent capacity fluid charge in order to mechanically latch the accumulator's piston, which may present an issue when frequent engine stop-start events occur.